Electro-discharge machining apparatus

Abstract

An electro-discharge texture machining apparatus includes an electrode carrier (10) movable towards and away from a workpiece surface and arranged to "scan" such surface. An electrode (12) on the carrier is supplied with short fixed duration high-voltage initiating pulses and longer fixed (but adjustable) duration low-voltage maching pulses each commencing with an initiating pulse.

I claim: 
     
       1. An electro-discharge machining control comprising first and second current sources of relatively high and low voltage respectively, first switch means for repeatedly connecting said second source to an output for a first duration for connection to an electrode and second switch means for repeatedly connecting said first source to said output only when said second source is connected to said output and for a constant duration which is shorter than the first duration of the connection of said second source to the output. 
     
     
       2. An electro-discharge machining control as claimed in claim 1 in which there are provided adjustable on and off time controls for said first switch means. 
     
     
       3. An electro-discharge machining control as claimed in claim 2, in which there are a plurality of said first switch means in series with resistors and a current selector means for determining the total discharge current by selectively operating said plurality of first switch means. 
     
     
       4. An electro-discharge machining apparatus for texture machining the surface of a metal roll comprising: roll support means for rotatably supporting a roll for machining;   a support structure movable in a direction parallel to the roll axis;   an electrode holder mounted on said support structure and movable relative thereto in a direction transverse to the roll axis;   a plurality of separate electrodes mounted on said electrode holder and electrically insulated from one another;   a plurality of first switch means for connecting said electrodes to a source of relatively high voltage;   a plurality of second switch means for connecting said electrodes to a source of relatively low voltage;   initiation pulse control means for repeatedly supplying relatively short constant duration initiation pulses to said first plurality of switch means so as to apply relatively short duration relatively high voltage pulses to said electrodes;   voltage detection means for detecting the voltage across gaps between the electrodes and the roll;   machining pulse control means for repeatedly supplying relatively long duration machining pulses to said second plurality of switch means so as to apply relatively long duration relatively low voltage pulses to said electrodes;   machining pulse inhibiting means, responsive to said voltage detecting means, for interrupting the supply of machining pulses to said second plurality of switch means in the event that said voltage detecting means detects a short circuit between an electrode and the roll; and   actuator means, responsive to said voltage detecting means, for displacing said electrode holder towards and away from the roll surface in accordance with detected discharge conditions.   
     
     
       5. Electro-discharge surface texture machining apparatus for providing a predetermined texture on a surface of a workpiece, comprising: at least one electrode holder;   at least one electrode mounted on said electrode holder;   a base structure;   a workpiece support;   means mounting said electrode holder and said workpiece support on said base structure for scanning movement of said electrode over a surface of a workpiece to be texture machined;   current pulse supply means for supplying pulses of machining current to said electrode, said current pulse supply means comprising:   (a) a source of relatively low voltage machining pulses of predetermined frequency and of predetermined duration;   (b) adjustment means for adjusting the frequency and duration of said machining pulses; and   (c) a source of relatively high voltage initiation pulses of a fixed duration, where said fixed duration is short relative to the machining pulse duration, said initiation pulses coinciding with the commencement of each of said machining pulses; and   means sensitive to discharge conditions for adjusting the proximity of the electrode to the surface.   
     
     
       6. An electro-discharge surface texture machining apparatus as claimed in claim 5 wherein said low voltage source comprises: a low voltage supply;   a first switch for connecting said low voltage supply to the electrode; and   a first timing circuit controlling said first switch; and said high voltage source comprises:   a high voltage supply;   a second switch connecting said high voltage supply to the electrode; and   a second timing circuit controlling said second switch, said second timing circuit being triggered into operation for said fixed duration each time the first timing circuit provides a control pulse to said first switch.   
     
     
       7. An electro-discharge surface texture machining apparatus as claimed in claim 6 in which said first timing circuit comprises: a presettable counter;   means for loading the counter periodically with count values representing the required on duration of the first switch alternatively with count values representing the required off duration of the first switch;   a clock pulse source for clocking said counter; and   means, sensitive to the counter count state, for alternately turning said first switch on and off.   
     
     
       8. An electro-discharge surface texture machining apparatus as claimed in claim 7 in which said second timing means comprises a monostable circuit triggered by said first timing means. 
     
     
       9. An electro-discharge surface texture machining apparatus as claimed in claim 8 in which said counter is a 16 bit counter, said clock provides clock pulses at a frequency of 1 MHz and said monostable circuit provides a 3 μS pulse duration. 
     
     
       10. An electro-discharge surface texture machining apparatus as claimed in claim 5 further comprising: gap voltage sensing means; and   machining pulse inhibition means for inhibiting a machining pulse in response to said gap voltage sensing means detecting a short circuit condition.   
     
     
       11. An electro-discharge surface texture machining apparatus as claimed in claim 6 in which said low-voltage source comprises: a plurality of said first switches; and   a plurality of resistor means for connecting said switches to said electrode and for providing a plurality of separately switchable machining current paths in parallel with one another.   
     
     
       12. An electro-discharge surface texture machining apparatus as claimed in claim 11 further comprising selection means for selecting different combinations of said first switch means for operation during said machining pulses, so as thereby to determine the level of machining current. 
     
     
       13. An electro-discharge surface texture machining apparatus for providing a predetermined texture on a surface of a workpiece, comprising: a base structure;   means supporting a workpiece on said base structure;   a carriage mounted on said base structure;   a plurality of electrode holders mounted on said carriage;   a plurality of electrodes mounted on said electrode holders;   transport means for said carriage and said workpiece support means for systematically scanning each of said electrodes over the whole surface of a workpiece to be texture machined;   a plurality of feed means for independently moving respective electrode holders relative to the carriage towards and away from the workpiece surface along spaced paths transverse to the scanning motion of the electrodes relative to the workpiece; and   means for supplying discharge current pulses to each of said electrodes;   each of said feed means comprising an independent servo-drive including:   a sensing device for sensing a discharge condition at the corresponding electrode; and   a closed loop control means for moving the associated electrode holder in response to said sensing device.   
     
     
       14. An electro-discharge surface texture machining apparatus as claimed in claim 13 in which said workpiece is a roll and said transport means includes means for rotating the roll about its axis and further means for displacing said carriage in a direction parallel to said axis, said feed means being arranged to displace the respective electrodes substantially radially relative to the roll. 
     
     
       15. An electro-discharge surface texture machining apparatus as claimed in claim 14 in which said electrode holders are spaced circumferentially around the roll axis in a plane perpendicular to the roll axis. 
     
     
       16. An electro-discharge surface texture machining apparatus as claimed in claim 13 in which there are a plurality of electrodes on each electrode holder, said electrodes being individually electrically insulated from the electrode carriers and each being associated with a separate independent machining current pulse supply.